The present invention is mainly intended for a tie rod connection on a plunger of a press, in which the plunger is driven for its respective stroke by a drive device by way of tie rods (also in combination with a connecting rod) and the tie rods are detachably connected to the plunger.
As defined by the present invention, the connection arrangement and its use are, however, also applicable to any machine tool such as work machines with stroke elements transmitting a force in an alternating direction in which occurring load changes particularly affect the connection arrangement.
Such machine tools or work machines can be designed as presses, punching presses, guillotine shears, scrap shears, etc.
Connection assemblies must be understood as machine elements which are exposed to load changes due to the stroke and which detachably or permanently connect the machine elements transmitting the force from the drive device to the stroke element, such as, for example, the plunger.
The present invention is aimed at a generic machine tool or work machine such as a press with a bottom drive, which includes:                a) at least one machine table with a tool receiving area for at least one bottom tool and at least one plunger (stroke element) with a tool receiving area for at least one upper tool, disposed above the machine table,        b) a drive device disposed in a sub-structure, which moves the plunger (stroke element) toward the machine table,        c) at least one connecting rod (traction/pressure element or tie rod) coupled to the plunger, which is directly or indirectly coupled to the plunger (stroke element),        d) at least one crank shaft rotatable about a shaft axis to which the connecting rod (traction/pressure element or tie rod) is coupled,        e) at least one motor or servomotor of the drive device, and        f) a connection arrangement between the connecting rod (traction/pressure element or tie rod) and the plunger (stroke element) by way of at least one traction/pressure point.        
Among the complex machine functions acting in this type of machine, the connection arrangement between the connecting rod (traction/pressure element or tie rod) and the plunger (stroke element) via the traction/pressure point presents particularities, as a functional group, which are highlighted and examined below.
DE 10 2011 016 669 A1 describes a press with a bottom drive connecting the drive-transmitting tie rods with the plunger, respectively, by means of a previously mentioned connection arrangement consisting of a screw and nut at a traction/pressure point of the plunger.
Implementing a tie rod connection with at least one tie rod, mounted at, respectively, one traction/pressure point of the plunger, with a thread located on one tie rod part and a nut, which is rotatable mounted in the plunger and is drivable by means of gears, has additionally already been proposed.
During normal operation of a press, the drive elements as well as the tie rods are exposed to a changing application of force, more specifically, due to the functions related to the load change caused by the strokes of the plunger, such as the reversals occurring between machining strokes and idle strokes.
During a machining stroke with a forming process, for example, the tie rods thus pull on a nut, wherein the lower thread flanks on the tie rod are then loaded. In contrast, during the idle stroke with an upward stroke of the plunger, the plunger rests with the force of its mass on the upper flank of the thread of the tide rod.
Due to the load changes occurring in the thread with each stroke of the plunger, these changes have a disadvantageous effect on the durability at least of the force transmitting elements but also on the noise behavior of the press.
These problems in general have a disruptive impact on any connection arrangement on a stroke element transmitting an original force in an alternating direction in a machine tool or work machine such as a press with a bottom drive. The main disadvantage is wear due to load changes on the involved machine elements.
Several concrete solutions are known to solve these type of problems.
DE 102007030772 A1 describes a plunger connecting device free of load changes in a press with an adjustment device which is pre-stressed, for example, by a hydraulic pre-stressing device with a force that is greater than the force to be transmitted by the adjustment device.
This is based on the fact that a change of sign of the load direction can occur in such adjustment devices during the back-and-forth movement of the plunger which has the following characteristics: in a bulk press, for example, with a plunger adapted to be moved vertically up and down, the load of the plunger and of the upper tool part normally hangs on the connecting rod and in addition on a so-called weight compensation device (SGA). During forming of the work piece, the connecting rods moving the plunger transmit a compressive force onto the plunger. Prior to that and after that they transmit a tensile force.
This load change leads to wear on the adjustment device which connects the connecting rod and the plunger (connection arrangement), wherein the threads available there are more specifically jeopardized.
This improved bulk press, in particular, with a plunger that is connected to a drive device by way of at least one connecting rod, comprises a pre-stressing device which is attached to one of the said adjustment devices and which avoids load changes on the adjustment device.
Any clearance in the area of the adjustment device is thus to be eliminated. Since there is no reversal of the direction of the load, but rather a dynamically increasing load, the durability of the elements engaged with each other is increased. The size of the thread of the adjustment nut and the adjusting spindle in the thread connection of the adjustment device can be chosen correspondingly smaller while having the same durability.
The remaining design consists in:                the thread connection having a first thread element that is connected to the connecting rod and a second thread element that is connected to the plunger and the pre-stressing device stressing the first thread element against the second thread element,        both thread elements being disposed concentrically to a force transmission direction,        the thread elements being pre-stressed against each other in the direction of the force transmission,        the pre-stressing device generating a force which is bigger than the greatest expected force to be transmitted by the connecting rod to the plunger during normal operation of the press,        the pre-stressing device generating a force acting between a thread element of the adjustment device that is connected to the connecting rod and the plunger,        the pre-stressing device having a piston that is adapted to be hydraulically loaded and rests on one of the thread elements in the axial direction, the piston being non-rotatably connected to the other of the thread elements or the adjustment device,        a rotary drive device being attached to the adjustment device, and        the pre-stressing device having an overload safety device or is connected to one.        
From this solution, the person skilled in the art learns, with regard to the functional group of a connection arrangement highlighted above, a pre-stressing device that is meant to avoid load changes on the adjustment device. The force to be applied for pre-stressing must, however, correspond to the pressing force which requires energetically as well as structurally complex solutions.
DE 000019706656 A1 describes a mechanical press comprising a frame, a bed fastened to the frame, a plunger referred to as a slide, which is connected to the frame in order to implement a back and forth movement relative to the bed. The plunger is driven by a crank shaft via a connecting rod. The plunger and the bed define the shut height of the press when the plunger is at the bottom dead center.
In accordance with the object, the shut height must be precisely and reproducibly adjusted.
A drive unit with a drive piston and tie rod and a shut height adjusting device with a chamber are provided for driving the plunger.
The shut height adjusting device comprises a pressure source which is in conductive connection with the chamber. By loading the chamber with a medium from the pressure source, the tie rod is lengthened or shortened for adjusting the shut height.
This solution teaches loading the chamber with a medium from a pressure source for adjusting the shut height in order to lengthen or shorten the tie rod (traction/pressure element), but without a pre-stressing function.
DE 41 18 569 describes a so-called hydraulic pressure point between a connecting rod and a plunger of a press for cutting, deep drawing or embossing. This pressure point can also be disposed between the table and the frame of a press or between other parts of a machine in a force flow. The present invention serves to prevent overturning, for cutting impact dampening or/and as overload protection and can be used as part of a force measuring system. To this end, two telescopic piston/cylinder assemblies are disposed in the force flow. These assemblies enclose two pressure chambers and are stressed against each other by the pre-stressing pressure in the chambers against a spring. A hydraulic switch between the pressure source and one of the chambers comprises a continuous valve, the difference between the actual pressure in the other chamber and its pre-stressing pressure being applied to its control input, said pre-stressing pressure corresponding to a positive difference between the actual pressure and the pre-stressing pressure of the control chamber and to an increase in pressure in the controlled chamber.
The person skilled in the art learns therefrom that a cutting impact dampening on the one hand and a pre-stressing pressure on the other hand is to be generated by way of a hydraulic switch, wherein, in terms of construction, no functional connection between cutting impact dampening and pre-stressing pressure, with regard to the hydraulic pressure point between the connecting rod and the plunger, is taught.
AT 008 633 U1 describes the following:
Hydraulic presses have a hydraulic cylinder between the table and the plunger moving relative to it. This simple hydraulic drive unit must nonetheless fulfill high safety requirements. As a solution, an energy store is provided, the stored energy of which is sufficient to generate a restoring force, which acts against the weight of the plunger and is greater than said weight. The weight of the plunger is thereby compensated for.
This simple press is not usable as a generic press as described above; it merely teaches the measure of applying a force that is greater than the weight of the plunger, which is well-known to developers of large presses.
WO 2010/072208 A2 describes a method for operating a forming machine or a forming unit with at least one plunger disposed on a shaft via an articulation. Hereby:                at least one rotary drive acting intermittently on the shaft provides a required forming energy to the plunger, and        at least one direct drive acting intermittently on the plunger allows for a defined movement of the plunger.        
The gravitational acceleration which effectively acts on the plunger is thereby adapted by way of the direct drive.
This method is characterized in that:                another direct drive, for example, a servo-motor, acting at least intermittently on the plunger allows for a defined periodic movement of the plunger,        the plunger experiences, by way of the direct drive, a counterforce reducing the gravitational acceleration in an upper stroke range,        the direct drive has a hydraulic or pneumatic configuration,        at a predetermined gravitational acceleration, the plunger engages with the rotary drive, for example, in the lower stroke range, or disengages from the rotary drive, for example, in the lower stroke range,        the plunger weight compensation device (SGA) is used as the direct drive adapting the gravitational acceleration,        the plunger performs an accelerated falling movement in an upper stroke range, the falling movement is influenced, for example, slowed down, by a force generated by the plunger weight compensation device, the rotary drive is engaged with the shaft at a time of a defined fall velocity synchronized with the speed of the rotary drive, the engaging process occurring, for example, in the lower stroke range, more specifically, shortly before forming, and the rotary drive being disengaged in the lower stroke range (shortly after forming) at a synchronized plunger velocity, and the dwell time of the plunger in the lower stroke range being reduced by a factor of at least 1:2 compared to the dwell time in the upper stroke range.        
Aside from the fact that this method does not address the connection arrangement defined in the introduction, it teaches that the direct drive adapting the gravitational acceleration uses the so-called plunger weight compensation device (SGA).
DE 10 2009 055 739 A1 describes the forming machine (also with a bottom drive) as a quasi-generic press.
DE 10 2009 055 739 A1 describes that the high forces occurring during pressing processes, which put a strong strain on the mechanical components and therefore limiting the efficiency, shall be acceptable for the press and a more simple construction should be provided.
This press comprises:                At least one machine table with a tool receiving area for at least one lower forming tool and at least one plunger with a tool receiving area for at least one upper forming tool, the plunger being disposed above the machine table and movable by means of a drive device relative to the machine table.        A drive device for the plunger with several connecting rods which are directly or indirectly coupled to the plunger in an upper connecting rod bearing.        At least one crankshaft that is rotatable about a shaft axis and to which at least one connecting rod is coupled in a lower connecting rod bearing.        At least one servo-motor for driving the crank shaft, each crank shaft and the lower connecting rod bearing of the connecting rods being disposed under the tool receiving area of the machine table and the upper connecting rod bearing of the connecting rod being disposed above the tool receiving area of the machine table and/or above the tool receptacle of the plunger.        
From the press of the type defined above, the person skilled in the art cannot learn any characteristics regarding the functional group of a connection arrangement of the connecting rod (traction/pressure element or tie rod) with the plunger (stroke element) via the traction/pressure point.
If the person skilled in the art examines, after this analysis of the prior art, the functional configuration and interrelationship of a pressure or traction point of a machine tool such as a large press with regard to the functional group of a connection arrangement of the connecting rod (traction/pressure element or tie rod) with the plunger (stroke element) via the traction/pressure point, they would discover that:                1. In forming presses with a bottom drive and linear movements of the plunger, different forces occurring during a plunger stroke are inevitable with regard to amount as well as direction for the continuous operation of the machine. During the downward movement, the work piece is essentially formed and thus presses against the plunger, whereas during the upward movement, the plunger is moved in the opposite direction and a force change occurs in individual components due to gravity.        2. On the other hand, in presses with a top drive, the weight is, as a rule, increasingly compensated for during the downward movement of the plunger by a corresponding arrangement of one or several pneumatic cylinders, so that all the components located in the force flow are already more or less pre-stressed before the actual forming process and a distinct and significant increase of the force occurs at the beginning and during forming, but without the force being changed in terms of direction.        3. The arrangement of the aforementioned plunger weight compensation device (SGA) serves to compensate for the weight of the plunger. In order to compensate for the weight of the plunger, it is well-known to install pneumatically pre-stressed cylinders between the frame of the press and the plunger, so that the weight of the plunger is absorbed by the cylinders of the SGA and the drive is not accelerated by the weight of the plunger during the downward movement. In return, the upward movement is not slowed down since the pressure in the cylinders of the SGA is adjusted so that it corresponds approximately to the weight of the plunger. A disadvantage is that the mass of the plunger, i.e., the weight caused by its speed, cannot be compensated for.        4. A relatively “soft” touchdown of the plunger on the component to be formed and on the counter-tool lying below it is thus always advantageous during operation of the press, an operation preserving the machine and the components being thus given. An abrupt force change can occur specifically during cutting operations, namely, when cutting through the piece to be cut, the entire machine thus unloading abruptly (the so-called cutting impact). In order to technically master this problem, there are extensive technical solutions which are known under the technical term “cutting impact dampening”. Said traction/pressure points are also designed so that they are pre-stressed, so that the adjustment within the traction/pressure point, which is required for the adjustment of the plunger and is implemented as a rule via a thread system, does not experience a change in force direction during these cutting impacts and the traction points have a longer durability.        5. In generic machines, the problem of an energetically and structurally advantageous use of a force that corresponds to the weight of the plunger, remains widely unsolved for the characteristics of a functional group of a connection arrangement of the connecting rod (traction/pressure element or tie rod) with the plunger (stroke element) via the traction/pressure point.        